Functional coating of the SCFM preform

ABSTRACT

The invention relates to a power semiconductor module having at least one semiconductor chip ( 11 ) made of a semiconductor material and having a first and a second main electrode ( 12, 13 ), a first and a second main connection ( 91, 92 ) and a contact lamina ( 2 ) in electrical contact with the first main electrode ( 12 ) and the first main connection ( 92 ). The contact lamina ( 2 ) contains an alloying partner which can form a eutectic with the semiconductor material. According to the invention, the contact lamina is coated with an electrically conductive protective layer ( 31, 32 ) that prevents formation of a fixed material connection between the first main electrode ( 12 ) and the contact lamina ( 2 ).

TECHNICAL FIELD

The present invention is concerned with the field of power electronics.It relates to a power semiconductor module according to the preamble ofthe first claim, in particular an IGBT (insulated gate bipolartransistor) or diode module.

PRIOR ART

In the case of power semiconductor modules, in particular in the case ofpower semiconductor modules which comprise at least one firstsemiconductor chip having an IGBT or diode structure internally, ashort-circuit strength is often intended to be ensured. In this case,short-circuit strength is understood to mean the fact that a stableshort circuit forms in the case of a defect in a first semiconductorchip between a first main connection of the power semiconductor module,which is electrically connected to a first main electrode of the firstsemiconductor chip, and a second main connection of the semiconductormodule, which is electrically connected to a second main electrode ofthe first semiconductor chip. In a resultant short-circuit mode, apermanent electrical contact with the lowest possible resistance and thehighest possible current capacity is intended to exist between the twomain connections. For this reason, the technical jargon in Englishincludes the term “short circuit failure mode”, SCFM for short. TheEuropean patent application published under EP 989611 A2 describes howsuch short-circuit strength is achieved with the aid of a suitablecontact element that is in contact with the first main electrode of eachsemiconductor chip. The contact element is formed by an electricallyconductive layer, for example a lamina, a slice or a foil, and mustcontain an alloying partner that can form a eutectic with asemiconductor material of the semiconductor chip, that is to say acompound or alloy whose melting point is lower than the melting point ofthe pure semiconductor material and at the same time lower than themelting point of the pure alloying partner. In the event of a defect inthe first semiconductor chip, the latter melts to form a eutectic withthe alloying partner and a metallically conductive channel forms betweenthe first and second main electrodes.

For semiconductor chips made of Si, in particular Al, Ag, Au, Cu or Mgor else a compound of these elements is suitable as alloying partner. Inthe case of pressure-contact-connected power semiconductor modules, afoil or a lamina may advantageously be placed as contact element ontothe first main electrode of each semiconductor chip and be fixed bypressure transmitted by means of contact plungers, by way of example.

In the case of a configuration of this type, however, problems can arisedue to the fact that a fixed material connection between first mainelectrode and foil or lamina forms over the course of time and can bereleased only with great expenditure of force, which generally entailsdestruction of the first main electrode and thus of the semiconductorchip. This is caused by the fact that the foil or the lamina is as itwere cold-welded to the first main electrode on account of the pressureexerted and on account of temperature fluctuations and cycles duringoperation of the power semiconductor module. During operation of thepower semiconductor module, different coefficients of thermal expansionbetween the contact element and the semiconductor chip result in asevere mechanical loading on the first main electrode, which may havethe effect that an electrode metallization becomes detached over thecourse of time, as a result of which the semiconductor chip isdestroyed. In unfavorable cases, this phenomenon may even occur just asa result of friction between the surfaces of the contact element and thefirst main electrode, that is to say without a fixed material connectionhaving formed.

A further problem arises in the case of power semiconductor modules notsealed in airtight fashion in conjunction with a contact element whichsubstantially comprises Al. It is generally known that Al forms an oxidelayer a few nanometers thick within seconds as soon as it comes intocontact with air at room temperature. Since said layer is insulatingand, moreover, generally harder than the electrode metallization of thefirst main electrode, it increases a contact resistance between thecontact element and the first main electrode, in the worst case to sucha great extent that, during operation of the power semiconductor module,heat generated by the contact resistance destroys the powersemiconductor module.

SUMMARY OF THE INVENTION

Consequently, it is an object of the invention to specify ashort-circuit-proof power semiconductor module having at least onesemiconductor chip which has a contact element in the case of which theproblems mentioned in the last section do not occur.

This and further objects are achieved by means of a power semiconductormodule of the type mentioned in the introduction having the features ofthe independent patent claim. Further advantageous refinements of theinvention are specified in the dependent claims.

The power semiconductor module according to the invention has, ascontact element, an electrically conductive contact lamina which iselectrically conductively connected to a first main electrode of thesemiconductor chip and a first main connection of the powersemiconductor module and which is coated with an electrically conductiveprotective layer. In this case the protective layer is preferablyconfigured such that it has, at an external contact area, a materialwhich

-   -   is as far as possible non-oxidizable, and preferably exhibits        little chemical reactivity, or    -   does not react chemically with a first electrode metallization        of the first main electrode and as far as possible exhibits        neither contact corrosion nor material diffusion, or    -   has a lowest possible coefficient of friction, or    -   can be deposited at temperatures at which the contact layer is        not damaged or deformed,        or which has an arbitrary combination of two or more of the        properties mentioned.

In a preferred development of the power semiconductor module accordingto the invention, the protective layer has a layered structure andcomprises at least one surface layer, which forms the external contactarea, and a base layer. In this case, the surface layer is formed from amaterial which has one of the properties mentioned in the last paragraphor an arbitrary combination of two or more of said properties.

These and further objects, advantages and features of the invention willbecome apparent from the following detailed description of a preferredexemplary embodiment of the invention in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically shows a cross section through a power semiconductormodule according to the invention.

The reference symbols used in the drawing and their meanings aresummarized in the list of reference symbols. In principle, identicalreference symbols designate identical parts.

WAYS OF EMBODYING THE INVENTION

FIG. 1 schematically shows a cross section through a power semiconductormodule according to the invention.

A semiconductor chip 11 having a first and a second main electrode issituated between an electrically conductive baseplate 91, which forms asecond main connection of the power semiconductor module, and anelectrically conductive covering plate 92, which forms a first mainconnection of the power semiconductor module. First and second mainelectrodes have a first and second electrode metallization 12 and 13,respectively. A contact lamina 2 is situated between first mainelectrode and covering plate 92, said contact lamina being coated with aprotective layer comprising a base layer 31 and a surface layer 32. Themodule is closed off by side walls 93, in which case this closure neednot necessarily be hermetic. In this case, a first thickness of thecontact lamina 2 preferably amounts to at least half of a secondthickness of the semiconductor chip 11. In this case, the contact lamina2 may advantageously be obtained by cutting out or stamping out from afoil. Preferably, however, use is made of an even thicker contact lam ma2 having a first thickness in the range of a few tenths of a millimeterto a few millimeters, preferably having a first thickness ofapproximately one millimeter. In this case, the contact lamina 2 mayadvantageously be produced by cutting out or stamping out from a metalsheet.

The contact lamina 2 preferably substantially comprises Al or Ag. Thesematerials are, on the one hand, relatively cost-effective. It hasemerged, moreover, that the formation of a eutectic is effectedparticularly reliably in the case of Si as semiconductor material withcontact lamina 2 made of Ag and Al and extends through the entiresemiconductor chip 11. This can be attributed inter alia to the factthat Ag and Al do not form any intermediate phases with Si. Intermediatephases are understood to be mixtures having a quantitative ratio withina certain bandwidth or else a strictly stoichiometric quantitativeratio, which are characterized by specific physical properties. Suchintermediate phases are relatively stable and impede a diffusionprocess, so that alloying through the semiconductor chip 11 is no longerguaranteed. However, the contact lamina 2 may advantageously alsosubstantially comprise Cu, Au or Mg or an alloy made of two or more ofthe metals Al, Ag, Au, Cu or Mg.

The surface layer 32 preferably substantially comprises a noble metal,advantageously Ag, Au, Pd, Rh or Ru. Preferably, the surface layer 32has a third thickness of 0.1 μm to 5 μm, preferably of approximately 0.2μm.

Preferably, the surface layer 32 may also substantially comprise anelectrically conductive nitride, advantageously TiN, CrN or ZrN, orgraphite. In this case, the third thickness is preferably 1 μm to 5 μm,preferably approximately 1 μm. Here and hereinafter “A substantiallycomprises X” is understood to mean that X is that substance having thelargest proportion by weight of all the substances X, Y, Z, . . .contained by the article A. In this case, X preferably has a proportionby weight of at least 90%. However, A may advantageously also comprisepure X.

In a preferred refinement of the invention, the contact lamina 2substantially comprises Al or Mg, and the protective layer has a baselayer 31 made of a good covering material, preferably chemicallydeposited or electrodeposited Ni. In this case, a fourth thickness ofthe base layer 31 is preferably a few micrometers, preferablyapproximately between 1 and 15 micrometers, preferably approximatelybetween 2 and 3 micrometers. In this case, the base layer 31 preventscontact corrosion between the contact lamina 2 and the surface layer 32.

In a preferred refinement of the invention, the surface layer 32substantially comprises Rh, Ru or an electrically conductive nitride,preferably TiN, CrN or ZrN. At customary operating temperatures, Rh hasonly very weak diffusion, and Ru and nitride even have no diffusion atall, at contacts with a first electrode metallization 12 made of Ag. Theformation of a fixed material connection between first electrodemetallization 12 and contact lamina 2 is thus prevented in aparticularly effective manner. If the contact lamina 2 substantiallycomprises Al or Mg and the surface layer 32 substantially comprises Ru,the base layer 31 is advantageously somewhat thicker, preferablyapproximately between 6 μm and 15 μm. This is important particularlywhen Ru is deposited in an Ru bath in which a pH value of approximately1 typically prevails, and which is highly chemically aggressive for thisreason. Moreover, a thin gold layer is advantageously provided betweenthe base layer 31 made of Ni and the surface layer 32 in order toimprove an adhesion between Ni and Ru. In this case, a fifth thicknessof the gold layer preferably lies in the region of a few tenths of amicrometer; it is preferably approximately 0.2 micrometer.

In a preferred refinement of the invention, the protective layercomprises only an individual layer, preferably comprising a noble metal,advantageously substantially comprising Ag, Au, Pd, Rh or Ru. In thiscase, a sixth thickness of the protective layer is preferably between0.1 μm and 5 μm, preferably approximately 0.2 μm. Preferably, thesurface layer 32 may also substantially comprise an electricallyconductive nitride, advantageously TiN, CrN or ZrN, or graphite. In thiscase, the sixth thickness of the protective layer is preferably between0.1 μm and 5 μm, preferably approximately 1 μm.

LIST OF REFERENCE SYMBOLS

-   11 Semiconductor chip-   12 First electrode metallization of the first main electrode-   13 Second electrode metallization of the second main electrode-   2 Contact lamina-   31 Base layer-   32 Surface layer-   91 Baseplate-   92 Covering plate-   93 Side wall

1. A power semiconductor module comprising at least one semiconductorchip made of a semiconductor material and having first and a second mainelectrodes, first and second main connections, a contact lamina inelectrical contact with the first main electrode and the first mainconnection, the contact lamina containing an alloying partner capable offorming an eutectic between the alloying partner and the semiconductormaterial, the contact lamina being coated with an electricallyconductive protective layer, wherein the protective layer has at leastone electrically conductive base layer applied on the contact lamina,and an electrically conductive surface layer, which forms an externalcontact area, the base layer and the surface layer substantiallycomprise different materials, and the surface layer is present betweenthe contact lamina and the first main connection and between the contactlamina and the semiconductor chip.
 2. The power semiconductor module asclaimed in claim 1, wherein the base layer comprises Ni and has athickness of approximately 1 μm to 5 μm.
 3. The power semiconductormodule as claimed in claim 2, wherein the thickness of the base layer isapproximately 2 μm to 8 μm.
 4. The power semiconductor module as claimedin claim 1, wherein the surface layer has a thickness of approximately0.1 μm to 5 μm.
 5. The power semiconductor module as claimed in claim 1,wherein the semiconductor chip internally has an IGBT structure or adiode structure.
 6. The power semiconductor module as claimed in claim1, wherein the base layer comprises a good covering material, and inthat the surface layer comprises a material having one or more of thefollowing properties: a non-oxidizable, exhibiting little chemicalreactivity, b does not react chemically with a first electrodemetallization of the first main electrode and exhibits neither contactcorrosion nor material diffusion, c has a low coefficient of friction, dcan be deposited at temperatures at which the contact layer is notdamaged or deformed.
 7. The power semiconductor module as claimed inclaim 1, wherein the surface layer encircles the contact lamina.
 8. Thepower semiconductor module as claimed in claim 1, wherein the surfacelayer encloses the contact lamina.
 9. The power semiconductor module asclaimed in claim 1, wherein the surface layer is between the contactlamina and the first main electrode of the semiconductor chip.
 10. Apower semiconductor module comprising at least one semiconductor chipmade of a semiconductor material and having first and a second mainelectrodes, first and second main connections, a contact lamina inelectrical contact with the first main electrode and the first mainconnection, the contact lamina containing an alloying partner capable offorming an eutectic between the alloying partner and the semiconductormaterial, the contact lamina being coated with an electricallyconductive protective layer, wherein the protective layer has at leastone electrically conductive base layer applied on the contact lamina,and an electrically conductive surface layer, which forms an externalcontact area, the surface layer substantially comprises Ru, anelectrically conductive intermediate layer is provided between thesurface layer and the base layer, said intermediate layer substantiallycomprising Au and having a thickness of approximately 0.2 μm, and thebase layer has a thickness of 5 μm to 12 μm.